Permeability of 16 Straight- and Branched-Chain Parabens Using the Caco-2 Assay.

Area of Science:

• Pharmacokinetics and Drug Metabolism
• In Vitro Toxicology
• Chemical Permeability Studies

Background:

• Limited understanding of in vitro intestinal permeability for straight- and branched-chain parabens.
• Parabens are widely used preservatives, necessitating investigation into their absorption and metabolism.
• Previous studies have not fully elucidated the transport mechanisms of various paraben structures.

Purpose of the Study:

• To investigate the in vitro intestinal permeability of sixteen different parabens.
• To differentiate between passive diffusion and active transport mechanisms for parabens and their metabolites.
• To explore the role of carboxylesterase-1 (CES1) and monocarboxylate transporter 1 (MCT1) in paraben transport.

Main Methods:

• Caco-2 cell monolayer assays to assess intestinal permeability.
• Parallel Artificial Membrane Permeability Assay (PAMPA) to measure passive diffusion.
• Transport studies using p-coumaric acid as an MCT1 substrate.
• Mass balance studies to account for paraben metabolism and non-specific binding.
• Kinetic measurements to determine transport rates and identify transporter involvement.

Main Results:

• Straight-chain parabens exhibited decreasing apparent permeability (Papp) with increasing chain length and LogP, consistent with passive diffusion.
• Hydrolysis of straight-chain parabens to 4-hydroxybenzoic acid (4-HBA) was observed, with hydrolysis extent inversely proportional to LogP, suggesting CES1 activity.
• Vectoral permeability (Papp A-B > Papp B-A) was noted for shorter straight-chain parabens, linked to active, pH-dependent efflux of 4-HBA via apical MCT1 transporters.
• Branched-chain parabens showed good passive diffusion but were poor CES1 substrates, indicating alternative transport pathways.
• Efflux of 4-HBA was primarily via the basolateral membrane.

Conclusions:

• Apparent permeability (Papp) values for parabens should consider both the parent compound and its metabolite, 4-HBA.
• The vectoral permeability of straight-chain parabens is influenced by hydrolysis extent and the interplay between passive diffusion and active efflux of parent compounds and 4-HBA.
• Branched-chain parabens may utilize transport mechanisms distinct from those involving 4-HBA and MCT1 efflux.